The present invention relates to radio-frequency identification (RFID) systems, including RFID tags and readers. The invention also relates to RFID apparatus and methodology that enable a plurality of items, such as items stacked on a pallet, to be read, including the innermost items in the stack, and even in the presence of RF intolerant material such as liquids and metals.
Automatic identification is the broad term applying to a host of technologies that are used to help machines identify items or objects such as cartons, boxes, bottles, and so on. Automatic identification is often coupled with automatic data capture. Therefore, companies wanting to identify items are able to capture information about the items, to store the captured information in a computer, and to retrieve selectively the information from the computer for a variety of useful purposes, all with minimal human labor.
One type of automatic identification technology is radio-frequency identification (RFID). Radio-frequency identification is a generic term for technologies that use radio waves to automatically identify items. There are several conventional methods of identifying items using RFID, the most common of which is to store a serial number (and other information, if desired) that identifies a product on a microchip that is attached to an antenna. The chip and the antenna together define an RFID transponder circuit. The antenna enables a remote reader that has a transceiver to communicate with the chip, and enables the chip to transmit identification information back to the reader when actuated to do so by the reader. The reader converts the radio waves returned from the RFID tag into a form that can then be utilized by a computer.
A conventional RFID system consists of a tag (including a chip with a antenna) and a reader (sometimes called an interrogator) with an antenna. The reader sends out electromagnetic waves that form a magnetic field when coupled with the antenna on the RFID tag. A passive RFID tag draws power from this magnetic field and uses the power to drive or activate the chip. The chip then modulates the waves that are sent back to the reader, and the reader converts the returned waves into digital information.
There are generally two types of RFID tags: active and passive. An active RFID tag utilizes a battery to power the chip and to transmit a signal to a reader (similar to a cell phone transmitting signals). A passive tag does not have a battery but rather is powered by the electromagnetic waves that induce a current in the antenna of the tag. A semi-passive tag uses a battery to power the chip but communicates by drawing power from electromagnetic waves from the reader.
Similar to a radio tuning to different frequencies, RFID tags and readers are tuned to the same frequency to communicate. RFID systems use many different frequencies, but the most common frequency ranges utilized in RFID systems are low-frequency (about 125 KHz), high-frequency (13.56 MHz), and ultra-high frequency or UHF (about 900 MHz). Microwaves, which have a frequency of about 2.45 GHz, are also used in some applications.
The distance at which an RFID tag can be read is known as the read range. The read range of a passive tag depends on a number of factors: the frequency of operation, the power of the reader, and interference from metal items or other RF devices. In general, low-frequency tags have a read range of about one foot; high-frequency tags have a read range of about three feet; and UHF tags have a read range of about 20 feet. Where longer read ranges are needed, an active tag with a read range of 300 feet or more can be used.
One of the desired applications of RFID tags is to track and inventory goods in a supply chain, particularly at high volumes such as a plurality of items stacked on a pallet on a loading dock or in a warehouse. One of the inherent difficulties in this application is ensuring that all of the RFID tags associated with all of the items in the stack are read, including the inner items of the stack that are obscured from view by the outer items of the stack. For example, if the stack of items is a five-by-five layer stacked five layers high (i.e., 125 items total), then the user would want to ensure that all 125 RFID tags are read, even those tags mounted to items located in the center of the stack.
The effectiveness of reading tags located in the center of a stack may be impaired by the presence of materials that are not particularly conducive to RF reading. More specifically, while all materials interact with RF waves to varying degrees, RF waves are able to travel through most non-metallic materials, so that RFID tags can be embedded in packaging or encased in protective plastic for weather-proofing and durability while still being readable. However, RF waves particularly reflect off of conductive materials such as metals and are particularly absorbed by certain other materials, such as water, fat, sugar, and protein at higher frequencies—absorptive materials that are commonly found in food items shipped in cartons. These characteristics make tracking metal products or those with high water content problematic. In addition, reading a stack of items with RFID tags, particularly items located in the center of the stack or items that may contain highly conductive or absorptive materials, is also problematic.
In view of the foregoing, there is a need in the art for RFID technology that enables a plurality of or all of the RFID tags in a stack of items to be read. The present invention satisfies this need.